This invention relates to heating elements and, more particularly, to ceramic, infrared-radiant heaters.
Heat transfer may be accomplished through convection, conduction and radiation. As is known, convection is heat transfer by mass motion of a medium such as air or water when the heated medium is caused to move away from the source of heat, carrying energy with it; conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole; and radiation is heat transfer by the emission of electromagnetic waves that carry energy away from the emitting object. Of the foregoing, radiation is the most efficient and flexible heat transfer means, and is adaptable to a variety of applications.
Industrial infrared heaters are generally classified by type (e.g., short, medium and long wavelength) based on the position of the maximum emission or peak wavelength in their spectral radiant power distribution. This categorization is based solely on the temperature of the heating element itself and by the application of Wien's displacement law. In other words, a short-wave heater is classified as such because its coil can reach steady state temperatures between 2148° F. (2 μm) and 6060° F. (0.8 μm); similarly, a medium-wave heater's coil temperatures is capable of reaching between 845° F. (4 μm) and 2148° F. (2 μm); and finally, a long-wave heater has coil temperatures less than 845° F. (or λmax>4 μm).
Radiant heating elements are typically used in applications where directional or focused heating is required. To this end, as is known, quartz heaters include elongated tubes and metal reflectors, and ceramic heaters are formed as curved or flat panels. Some processes used to manufacture heaters limit the shapes that the heaters may assume. Processes have been developed to produce heaters having non-standard shapes, but such processes have limitations on the internal construction of such heaters. These limitations on internal construction do not provide a heater having the highest potential efficiency. Yet other processes only allow for the production of a single type of heater (i.e., the process is capable of only producing a heater that radiates in a 180° range or a heater that radiates in a 360° range, not both).
Infrared radiation is absorbed by organic molecules and converted into molecular vibration energy. When the radiant energy matches the energy of a specific molecular vibration, absorption occurs. In one embodiment, an efficient infrared heating system comprises a set of infrared heaters with the emissive wavelengths finely tuned to match the absorption wave-lengths for a given application at its various stages of the heating process. That is, as the drying process progresses and the absorption wavelength of the material changes, the emissive wavelength changes accordingly, as shown in FIG. 1.
Referring to FIG. 1, Zone A of the system, near the entrance of the conveyor system, or process path, may contain short-wave heaters operating at near 2 μm to match the first peak of the absorption spectra for water (around 95%). In the middle of the heating application (i.e., Zone B), medium-wave heaters may be employed to match the second highest absorption peak (around 94%). Finally in Zone C, close to the end of the conveyor, just before exiting the system, and to prevent a strong thermal shock for the application material, long-wave heaters may be placed to match the final high absorption peak (around 78%).
In a real-world application, however, the construction and operation of such a system is very difficult to achieve because there is no infrared heater in the industry that can deliver short, medium or long waves as a single unit. Each heater type has unique design, construction and operation requirements that make them very difficult to combine with other types. For instance, the heat output of a short-wave emitter is so high that often cooling systems are required to maintain the heater's housing at permissible levels.
Currently used industrial radiant heaters have two elements in common, a reflective surface and a housing. Heaters provided by Elstein-Werk M. Steinmetz GmbH & CO. KG (Germany) and Heraeus Noblelight Inc. (Duluth, Ga.) both include a gold reflective material directly applied to the housing and to the quartz material, respectively. The direct application of the gold makes the overall size of the heater smaller and easier to handle because there is no need for a reflector (i.e., the body itself is a reflector). However, the power generated by the heated element cannot exceed a certain limit that would cause the gold to evaporate (greater than 820° C.). Further, there still is a considerable amount of heat that the reflector will absorb and conduct to the back-side of the heater, thereby heating up the structure that holds the heater and not the application. Heaters by Fostoria Industries (Fostroria, Ohio) and the Research Inc. (Eden Prairie, Minn.) require a reflector embedded in a steel housing for the heater to operate properly.
Another example of an industrial radiant heater includes a ceramic infrared heater that is either solid or hollow. High powered hollow heaters exhibit a tendency to develop cracks at the outer shell as a result of thermal expansion mismatch between an embedded coil layer and an outer shell. In simple heat transfer terms, the Joule heating generated at the coil is transferred to the surrounding ceramic layer by conduction. Because of the low thermal conductivity of ceramics, the coil layer is impacted significantly faster than the outer shell resulting in a large temperature gradient between both layers, causing at the same time, a large thermal expansion mismatch. In some cases, the tensile strains exceed the strength of the body and visible cracks develop to release the strain. These cracks form in either glazed or unglazed ceramic bodies, and those with or without heads. Such cracking suggests that the cracks were not induced by residual stresses caused by the cooling glaze, but rather by the larger expansion suffered by the coil layer during energization.
The challenge of designing an infrared heater that would emit in all available wavelengths requires consideration of the parameters of existing infrared units. Existing ceramic body heaters with embedded ferritic alloys (FeCrAl) have a high mechanical stability, but have maximum power limitations resulting in microstructure fractures that induce dielectric failure in high wattage/voltage units. Infrared heaters with quartz tubes enclosed in sheet frame have a resistance coil that freely expands within the tubes; however, the sheet metal structure is highly susceptible to corrosion, distortion and deformation. Finally, tungsten-halogen and carbon infrared lamps have a fast response time and provide control and management of the emitter wavelengths, but such lamps have limited assembly options.